1. Field of the Invention
The invention relates to voltage regulators, and more particularly to very low quiescent current voltage regulators used for powering very low power devices.
2. Description of the Related Art
Notebook computers are quite common today and becoming both more prevalent and more powerful. As the capabilities of the computers increase, generally so does the amount of space required for the components. As notebook computers have general size limits, this results in increased packing densities and very complex internal arrangements. As a result, except for certain desired components, the various units comprising the circuitry of the computer become very difficult to access. One cannot simply remove the cover by loosening a few screws and access the components. Disassembly is a very delicate and time consuming task, and reassembly is the same. Thus it is desirable to reduce the need to access any internal components.
One item standard in notebook computers is a real time clock (RTC)/CMOS memory section. This section provides a clock and continuous memory capability even if the computer is turned off and/or the main battery is removed. To this end a separate battery, conventionally a lithium cell for size and life reasons, is used to provide a power source to the RTC/CMOS memory section when the computer is turned off. Thus the operations can continue as necessary. However, it has been determined that in many instances the useful life of the RTC battery may be a relatively short period such as two years. After that time the battery cannot provide sufficient energy to allow the RTC/CMOS memory section to properly operate and configuration and date/time errors begin occurring. Replacement of the RTC battery is then necessary. But this RTC battery is not one of the few accessible components and so disassembly and reassembly of the computer is necessary for replacement. As noted above, this is undesirable for numerous reasons. Therefore is desirable to lengthen the effective life of the RTC battery.
At first blush, it might be considered straightforward to simply provide a separate voltage regulator from the notebook computer's main battery to provide power when the computer is turned off and the main battery is present. However, known voltage regulators have a very high quiescent current draw, such as 1 mA. This is unacceptable, particularly when the RTC/CMOS memory section draws only microamperes of current. The high quiescent current will relatively rapidly drain the main battery, thus resulting in a dead computer, even though it was turned off in a fully charged state. Therefore this solution to the RTC battery life problem is not practical using conventional voltage regulators. Thus a low quiescent current voltage regulator is needed to allow use of the main battery to power the RTC/CMOS memory section instead of the RTC battery.